This invention relates to material storage measuring devices and, in particular, to an advanced signal processing technique for accurately measuring the depth of a low dielectric material in a storage device.
Generally, a material monitoring device is a device which is intended to view the contents of a storage tank from above and report the distance to the top surface of what ever is stored in the tank. The device does this by, for example, measuring or estimating the frequency difference between transmitted and the received signals, which is proportional to the distance to the reflecting surface. In a correctly designed device, the situation is such that the difference frequency (IF) signal is either a sum of sinusoids or a sum of sinusoid modulated by the modulation signal.
However, several problems are associated with estimating the frequency which corresponds to the reflecting surface of interest. First and most foremost, if the material to be monitored is non-conducting and has a low dielectric constant, then the RF energy can penetrate the fluid and be reflected from whatever is below the surface of interest that is opaque to the RF signal. This could be the bottom of the tank, or it could be some other fluid (such as water for example) that is opaque to the RF signal. This results in a target which complicates the process of estimating the frequency of interest. To make matters worse, the RF power returned from the surface of interest can be as much as 30 dB less than that returned from the bottom of the tank. Accordingly, the signal of interest is much smaller than other returned signals. To make matters still worse, as the surface of the dielectric fluid approaches the bottom of the tank, the frequencies of the sinusoids resulting from these targets tend to converge, and can become very close. This is the so called "low dielectric problem" that has plagued material monitoring device vendors since the inception of such devices.
Second, if the modulation waveform is repetitive and the IF signal is observed over more than one cycle of the modulation, special processing must be done to separate the information bearing sinusoid from the harmonics of the modulation.
Third, there may be reflections from objects in the tank other than the reflecting surface of interest. These reflections contribute to the IF signal and must be appropriately dealt with in the process of estimating the distance to the surface of interest.
Fourth, the users of material monitoring devices require very accurate estimates of the distance to the surface of interest. This implies that the parameters of the modulation signal must be precisely known, and the estimate of the frequency of the sinusoid of interest must be precise.
Fifth, the Federal Communications Commission (FCC) requires that the RF signal radiated from devices intended for this application be limited both in power and bandwidth. The power limitation limits the signal to noise ratio that can be obtained in the IF signal. The band width limitation is the more severe handicap, and, in fact, the material monitoring devices sold by most vendors do not adhere to the FCC frequency limits, which means that the users of these devices must apply for a license that is specific to the site at which the device is to be used.
Hence, there exists a need for an improved technique for measuring the depth of a low dielectric material in a storage tank while not suffering from the problems discussed above.